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Table of Contents
Title- Which truss bridge will hold the most weight.
Broad Question- Which bridge can hold the most weight.
Specific Question- Which truss bridge, subdivided Warren truss, the Pratt Camelback truss, and the Howe truss, will hold the most weight before breaking.
Variables
Independent Variable: Balsa Wood
Dependent Variable: The Bridge
Variables That Need To Be Controlled: The five gallon
Hypothesis- I think that the Pratt truss will hold the most weight because it has a very nice superstructure and strong bars.
Graph of Hypothesis-
Experimental Design
General Plan- I am about to conduct an experiment that will determine which truss bridge, a Pratt, Warren, or Howe, will hold the most weight. My experiment will take all of probably a couple weeks to build the bridges and conduct the experiment. I will be conducting this experiment at home. My dad and I will build each bridge with balsa wood and Popsicle sticks. What I will do is hook a five gallon bucket to the bridge and if it can hold the bucket than we will gradually put a cup of water in the bucket at a time and when it breaks we will see how much water is in the bucket and record it.
Materials List
Balsa Wood
A box of Popsicle sticks
A bottle of glue
A five gallon bucket
Water
A bridge platform
Background Research
References- Macaulay, David. "Bridges." Building big. Boston: Houghton Mifflin, 2000. 9-52. Print
Boon., Garrett. "Model Bridge Design." Model Bridge Design. N.p., n.d. Web. 8 Feb. 2012. <http://www.garrettsbridges.com/>"Bridge Basics - A Spotter's Guide to Bridge Design." Bridges and Tunnels of Allegheny County and Pittsburgh, PA - pghbridges.com. N.p., n.d. Web. 8 Feb. 2012. <http://pghbridges.com/basics.htm>
The Warren Truss was patented by James Warren in 1848. It has been around a while. The Warren Truss uses equilateral triangles to spread out the loads on the bridge.The equilateral triangles minimize the forces to only compression and tension. Interestingly, as a load (such as a car or train) moves across the bridge sometimes the forces for a member switch from compression to tension. This happens especially to the members near the center of the bridge.The Warren Truss is simple but strong.
The Howe Truss was designed by William Howe in 1840. It used mostly wood in construction and was suitable for longer spans than the Pratt truss. Therefore, it became very popular and was considered one of the best designs for railroad bridges back in the day. Many Howe truss bridges exist in the North West United States, where wood is plentiful.
The Pratt and Howe trusses are very similar; the only difference is the way the beams are angled. On the Pratt truss, the shorter, vertical members are in compression. However, on the Howe truss, the longer, angled members are in compression. Most believe the Pratt has the bigger advantage.
The Pratt Truss was designed by Thomas and Caleb Pratt in 1844. It became popular for railway bridges because it made good use of iron."The Pratt Truss is one of my favorites. I have used it often for my model bridges, including balsa, basswood, and popsicle sticks. It is easy to construct, and is a solid choice for a model bridge design."
Results
Data Table-
Graphs
Photos
Data Analysis
Conclusion
Discussion